Traditionally, in an image forming apparatus such as a laser printer, there are known a type which a toner image formed on a surface of a photosensitive drum is transferred to a sheet transported by a sheet transport belt, and a type which a toner image formed on a surface of a photosensitive drum is transferred once to an intermediate transfer belt and then transferred to a sheet.
A belt such as the sheet transport belt or the intermediate transfer belt is wound between a driving roller input with a driving force and a driven roller spaced away from the driving roller at a predetermined interval, and contacts the surface of the photosensitive drum. Therefore, a toner and a sheet dust adhere to a surface of the belt when the belt contacts the photosensitive drum and a sheet respectively.
Accordingly, in an image forming apparatus having such a belt, a cleaning roller is provided and opposed to the surface of the belt. The belt is cleaned by generating a potential difference between the cleaning roller and the belt and transferring adherents on the surface of the belt to the cleaning roller by static electricity.
In order to preferably transfer the adherents on the surface of the belt to the cleaning roller, an electric current having a proper value is required to run through the cleaning roller. However, when the electric current applied to the cleaning roller is controlled (electric current control) such that a constant electric current runs through the cleaning roller, the potential difference between the belt and the cleaning roller becomes excessively large in case where a resistance value of the cleaning roller increases due to an influence of a use environment or deterioration over time. As a result, the belt may be broken (by surge).
Therefore, an electric voltage applied to the cleaning roller is generally controlled (voltage control) such that a constant potential difference is generated between the belt and the cleaning roller. However, with this voltage control, when the resistance value of the cleaning roller increases due to the influence of a use environment or deterioration over time, an electric current running through the cleaning roller becomes excessively small, so that it is impossible to preferably transport the adherents on the surface of the belt to the cleaning roller.